Osmosis is a known phenomenon in which water moves across a semi-permeable membrane between solutions with lesser and greater concentrations. In forward osmosis, the water moves from the lower concentration solution to the higher concentration solution, while in PRO osmosis the rate of flux of water can be reduced. By applying sufficient pressure to the higher concentration side, osmotic water flow can be reversed and water caused to move across the semi-permeable membrane from the higher to the lower concentration solution. The techniques have found use in a number of fields, including water treatment and desalination. Pressure-retarded osmosis has also been applied in power generation, where the pressure in a saltwater solution is increased by osmosis from a freshwater source, and the pressure is used to drive a turbine.
In known osmosis systems, it is known to use spiral-wrapped osmosis elements, each of which is comprised of a plurality of basic multiple-layer construction blocks in a repeated order. A typical such block comprises a single sheet of semi-permeable membrane which is folded over a feed sheet. The area of the semi-permeable membrane sheet is usually twice the area of the feed sheet spacer. Once the semi-permeable membrane is folded over the feed sheet spacer, the feed sheet spacer separates the two leafs of the folded semi-permeate membrane sheet. The feed sheet is configured to allow free solvent flow along the semi-permeable membrane's surface. The semi-permeable membrane is designed to prevent solute flow. This sandwich of folded semi-permeable membrane with its internal feed spacer is then interposed between spaced permeate spacers. The permeate spacers' area is usually bigger than the area of the folded leafs of the semi-permeable membrane in one dimension, so that once the folded semi-permeable membrane and associated feed spacer is interposed between spaced permeate spacers, a tail consisting of permeate spacers only extends from the interleaved permeate spacers and semipermeable membranes. Feed water supplied across the membrane will flow along the feed spacers contacting the solute rejection skin of the semi-permeable membrane. Multiple basic blocks like that, are spiral-wrapped around a central tube in such a way that each permeate spacers tail drains the water product produced by each basic block into a central tube through dedicated side holes. Feed water is forced to move generally along the longitudinal dimension of the wrapped feed spacers which is parallel to the tube, and permeates flow through the semi-permeable membrane into the permeate spacers and continue in spiral direction toward the central tube.
Spiral wound membranes in the prior art are designed to operate in a reverse osmosis process in which pressurised salt feed water is dewatered and produces fresh water product. In order to function in other applications which require high membrane area to volume, such membranes require some modifications as taught in U.S. Pat. No. 4,033,878. This document teaches a barrier located inside the central tube to force fluid flow to flow out from the central tube to the permeate spacers of the spiral wound membrane through the dedicated side holes. A unidirectional serpentine flow path within the permeate spacer is formed by a flow blocking glue line which extends within the permeate spacer in a direction which is generally perpendicular to the longitudinal axis of the central tube. This additional glue line divides each basic block of the spiral wound as described above, into two zones. All fluid flow in the central tube is blocked and forced out of the central tube into the permeate spacers' tail in the first zone, and flows in a serpentine pattern around the blocking glue line and back to the central tube to a point beyond the blocking means, and is drained back to the central tube through the permeate spacers' tail of the second zone. U.S. Pat. No. 4,033,878 further teaches a train of such osmosis elements connected in serial connection. According to U.S. Pat. No. 4,033,878, in any such element in the train of elements, the entire flow within the first portion of the central tube prior to the blocking means must be shunted out and flow through the spiral wound membrane to reach the second portion of the central tube beyond the blocking means. This serial path causes strong pressure drops and unequal pressure and flow distribution between the elements which limit the system's ability to support high flow rates.
U.S. Pat. No. 8,354,026 shows an improvement to this serial configuration by implementing a perforated vertical blocking means across the central tube. Small internal tube-shunts passing through the blocking means allow parallel flow through all membrane elements connected along a common central tube. This requires multiple tube-shunts which must be accommodated within the central tube diameter. The internal tube-shunts are arranged in an alternating configuration, each bypassing a blocked section of the central tube. As a result there are many sudden diameter contractions and sudden diameter expansions along the fluid flow path. Once the fluid leaves the main central tube and enters a tube-shunt there is a sudden drop in diameter along the flow path and once the fluid leaves the tube-shunt and enters the next blocked section of the central tube there is a sudden diameter expansion. This may cause strong pressure losses and again unequal flow distribution between osmosis elements along a common central tube line. Moreover, the usage of multiple tube-shunts is an ineffective usage of the cross section of the main central tube. In order to accommodate two parallel tubes within the central tube, their maximum diameter should be less than half of the central tube diameter. As a result, the combined area of the cross section of each tube-shunt is much smaller than the area of the cross section of the central tube.